1. Field of the Invention
The invention relates to pressure swing adsorption processes. More particularly, it relates to enhanced recovery of the readily adsorbable component of gas mixtures treated in such processes.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes, such as PSA-hydrogen, PSA-oxygen and PSA-methane, are commonly carried out by (1) adsorbing the readily adsorbable component of gas mixtures at high component pressure, (2) depressurizing the adsorption zone to effect desorption and an increase in the readily adsorbable component in the gas phase composition at the feed end of the adsorption zone, (3) removing the thus-enriched gas at the feed end of the adsorption zone from said zone by further depressurization or by purging with a purge fluid, and (4) repressurizing the adsorption zone or bed to its original condition. Such removal of the readily adsorbable component results in an accumulation of the less readily adsorbable component in the adsorption zone. This component can be removed from the adsorption zone during the high pressure adsorption step, during the depressurization step or following the purge step.
Enhanced purity of the readily adsorbable component cannot be achieved simply by equilibrating the adsorbent and the feed gas. A purer adsorbed phase can be obtained, however, by incorporating an intensification step after the adsorption step. For this purpose, a so-called copurge gas, constituting a pure gas having essentially the same composition as the readily adsorbable component, is introduced to the adsorption zone or bed at substantially the same pressure as is employed in the adsorption step. The pure readily adsorbable component displaces the less readily adsorbable component from the adsorption bed, thus resulting in a purer adsorbed phase and enhanced purity of the readily adsorbable component recovered upon desorption at a reduced pressure. Such an approach is described in the Tamura patent, U.S. Pat. No. 3,797,201, which discloses the use of recycled, readily adsorbable component for the copurge step carried out at the adsorption pressure. The amount of said recycled component, repressurized from low pressure product, constitutes a significant operating cost associated with this approach.
Another approach involving adsorption, copurge and desorption is described in the British Pat. No. 858,059. In the process of this patent, a molecular sieve is used to adsorb normally liquid, readily adsorbable components from mixtures thereof with less readily adsorbable components. A copurge step is carried out under conditions such that substantially the only normally liquid, readily adsorbable components remaining in contact with the sieve at the end of the copurge step are those adsorbed in the initial adsorption step. Such normally liquid, readily adsorbed components are then desorbed and recovered. The purging step is carried out by passing a normally gaseous material, such as isobutane and preferably nitrogen, over the molecular sieve. The purging step serves to remove non-adsorbed and surface-adsorbed material substantially without desorbing the readily adsorbable, normally liquid material from within the pores of the sieve. Purging conditions are said to be selected to reduce the tendency for material to be desorbed from within said pores, and include (1) the use of temperatures not higher than that employed in the adsorption step, (2) elevated pressure up to 150 p.s.i.g., with adsorption pressures of 0-150 p.s.i.g., preferably 50-100 p.s.i.g., and purging pressures of 0-150 p.s.i.g., preferably 0 p.s.i.g., being disclosed, or (3) reduced pressure, i.e. vacuum purging again preferably at a temperature not exceeding that of the adsorption step and for short periods not longer than that of the adsorption step. While this process is disclosed as providing improvements in the separation of straight chain hydrocarbons from mixtures thereof with branched and/or cyclic hydrocarbons, it does not relate directly to the separation of readily and less readily adsorbable components of normally gaseous mixtures, such as the separation of nitrogen from oxygen and the separation of methane from mixtures thereof with nitrogen. The requirement of copurging with a separate, normally gaseous material likewise adds to the overall cost and complexity of the process.
There remains, therefore, a need in the art for improvements in pressure swing adsorption processes for the separation of the readily adsorbable components of gas mixtures from the less readily adsorbable components thereof. Such improvements are particularly needed for use in pressure swing adsorption applications in which it is desired to recover the readily adsorbable component at enhanced purity levels. While PSA processes are normally employed to optimize the purity level of the less readily adsorbable component, some improvement in the purity level of the recovered readily adsorbable component can be achieved provided that the purity restriction for the less readily adsorbable component is removed. The PSA processes have been unable, however, to achieve desirable high purity levels for the readily adsorbable component despite efforts to optmize such levels within the scope of conventional processing. For example, a conventional PSA air separation process normally used to produce high purity oxygen, i.e. the less readily adsorbable component, has been found to produce nitrogen, the readily adsorbable component, at a purity level of about 86% using a particular adsorbent in a particular, 3-bed system. When this system is optimized for nitrogen recovery, it has been found that the nitrogen purity is increased to about 88%. The conventional process is unable to achieve enhanced nitrogen recovery, however, as to recover nitrogen at purity levels in excess of 95%.
The desired improvements in pressure swing adsorption processes, enabling the readily adsorbable component to be recovered at enhanced purity levels, would enable reasonably pure nitrogen or other inert gases to be produced in locations or applications where conventional inert gas generators or cryogenic air separation units are not suitable. For example, locations on board trucks or aircraft or ships, or applications involving small and/or intermittent use are not suited for inert gas production by conventional means. In addition, a need exists for a process for producing high Btu methane gas from mixtures thereof with low Btu nitrogen in natural gas wells that would be competitive with available cryogenic methods.
It is an object of the present invention, therefore, to provide an improved pressure swing adsorption process.
It is another object of the invention to provide a pressure swing adsorption process for the enhanced recovery of the readily adsorbable component of a feed gas mixture.
It is another object of the invention to provide a pressure swing adsorption process for the recovery of nitrogen of enhanced purity from air.
It is another object of the invention to provide a pressure swing adsorption process for the recovery of methane of enhanced purity from mixtures of methane and nitrogen.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being pointed out in the appended claims.